In recent years, the development of illumination sensors and ambient light sensors to be used for information terminal devices, such as smart phones and tablet terminals, has been proceeded. In information terminal devices, illumination sensors are used for detecting the illuminance of environments around the information terminal devices and controlling the brightness of displays, and ambient light sensors are used for adjusting the color tones of displays.
To match human visibility and the luminance and the color tone of a display in a natural manner, it is important that only visible light is allowed to reach an ambient light sensor. For example, when an ambient light sensor is provided with an optical filter such as a near-infrared cut filter, the spectral sensitivity properties of the sensor can be closer to the visibility.
On the other hand, a request to put higher importance on the design of information terminal devices have led to a required for reducing the transmittance of a transmission window through which light enters an ambient light sensor (a required for making the window have a blackish appearance), and accordingly, a problem has arisen that the amount of incidence of visible light with respect to infrared light decreases, and accurate detection of illuminance and color tones becomes difficult, whereby a malfunction occurs. Furthermore, information terminal devices become lower in height, whereby the distance from a window for light incidence to an ambient light sensor is shorter, and accordingly, for example, the ratio of incident light at a high incident angle such as an incident angle of degrees increases, and it has been required that, even in the case of incident light at a high incident angle, the spectral properties (particularly, the intensity of near-infrared rays) of light that reaches the ambient light sensor do not change.
As a means for matching the spectral properties of an ambient light sensor and human visibility, there has been disclosed a device provided with an infrared cut filter formed of a multilayer metal film on a glass plate (for example, refer to Japanese Unexamined Patent Application Publication No. 2011-060788). However, the optical properties of such near-infrared cut filter formed of a multilayer metal film on a glass plate greatly changes with the incident angle of incident light. Therefore, a problem has arisen that the detection accuracy of the ambient light sensor decreases.
On the other hand, various near-infrared absorbing particles are known as a means for blocking broadband near-infrared rays, independently of incident angles (for example, refer to WO 2005/037932 and Japanese Unexamined Patent Application Publication No. 2011-118255). To achieve sufficient near-infrared blocking performance for an ambient light sensor by using these near-infrared absorbing particles, a larger amount of the near-infrared absorbing particles needs to be added. However, there is a problem that a larger amount of the near-infrared absorbing particles added to a near-infrared cut filter causes a decrease in visible light transmittance.
By contrast, a near-infrared cut filter including a norbornene-based resin substrate, a near-infrared absorbing dye having the absorption maximum at a specific wavelength, and a near-infrared reflection film has a property that a change in transmittance in the visible range when light enters the filter from an oblique direction is smaller (for example, refer to Japanese Unexamined Patent Application Publication No. 2011-100084). However, to use this near-infrared cut filter for an ambient light sensor, it is desirable that the performance of blocking infrared rays at a high incident angle such as an incident angle of 60 degrees is further improved.